Turbochargers are of course well known devices which include a compressor or blower wheel, typically an impeller, which is situated in an engine inlet duct and is connected to an exhaust turbine, which is situated in the engine exhaust duct and arranged to be rotated at high speed by the engine exhaust gases. Rotation of the exhaust turbine results in rotation of the blower wheel which produces a boost pressure, that is to say it increases the pressure in the inlet duct to a superatmospheric value. The result of this increased inlet pressure is that a greater amount of air is admitted into each cylinder of the engine during the induction stroke of the pistons in the cylinders, which results in an increased power output from the engine.
The power absorbed from the exhaust gases by a turbocharger exhaust turbine is proportional to the cube of the speed of the exhaust gases, which means that although the blower wheel rotates very rapidly and thus produces a substantial boost pressure at high engine speed, it does not rotate at all or only at negligible speed at low engine speed. This means that no boost pressure is available at a time when maximum engine power is frequently needed, i.e. when accelerating rapidly from engine idle speed.
One way of overcoming this problem is to increase the speed of the exhaust gases past the exhaust turbine. This can be done by providing guide vanes of variable pitch in the exhaust duct to enable the local exhaust gas speed to be increased and thus the power output of the turbine wheel to be increased, even at low engine speed. However, such a construction is complex and expensive and subject to failure as a result of lubrication problems. Simply making the turbocharger physically smaller, thereby increasing the exhaust velocity through it, would substantially improve the characteristics of the turbocharger at low engine speeds, but at high engine speeds the exhaust turbine would constitute an unacceptable flow restriction for the exhaust gases and would be liable to failure as a result of being driven at an unacceptably high speed.
It has been proposed that an automotive engine be provided with a turbocharger system comprising two turbochargers, one relatively small and the other relatively large. The two blower wheels are provided in series in the engine inlet duct and the two exhaust turbines are provided in series in the exhaust duct. Since the small turbocharger is inappropriate at high engine speeds and would be liable to failure if used at such speeds, the smaller exhaust turbine and the smaller blower wheel are provided with respective bypass passages incorporating respective shut-off valves operated under the control of the engine management system.
The operation of such a system is supposed to be as follows: The two bypass valves are shut at low engine speeds. The relatively small volume of exhaust gas flows through the exhaust turbine of the smaller turbocharger at a substantial speed due to the relatively small dimension of the duct in which the turbine is situated. The smaller exhaust turbine is thus rotated at a substantial speed and this rotation is transmitted to the smaller blower wheel, which thus creates a significant boost pressure in the inlet duct. The exhaust gas also flows through the exhaust turbine of the larger turbocharger, but at a significantly lower speed due to its greater size. The larger exhaust turbine is thus rotated very slowly, if at all, and the larger blower wheel thus plays effectively no part in the creation of the boost pressure. As the engine speed and/or load rises, the engine management system opens the two bypass valves. The exhaust gas now flows through the passage bypassing the smaller exhaust turbine and then flows through the larger exhaust turbine where it now reaches a substantial speed due to the increased flow rate of exhaust gas. The larger exhaust turbine is thus rotated at high speed and this rotation is transmitted to the larger blower wheel, which creates a boost pressure in the inlet duct. The bypass duct around the smaller blower wheel has larger flow area than that of the smaller blower and thus does not constitute an unacceptable flow restriction in the inlet duct.
Accordingly, such a composite turbocharger system should provide a solution to the problem of inadequate boost pressure at low engine speeds. However, it is found in practice that it does not do so and tests have indicated that an engine fitted with such a turbocharger system has a power output of only about two-thirds of that which would be expected at low engine speeds.
In addition difficulties are encountered in controlling operation of the individual turbochargers and in particular airflow. For example the larger turbocharger has a turbine bypass valve (for bypassing the larger turbine in an overboost or overspeed condition) and control of the smaller turbine bypass and larger turbine bypass must be achieved without competition between the control strategies. Yet a further problem is that the smaller compressor can act as a restriction on airflow from the larger compressor whilst producing no pressure rise at higher engine speeds/loads.
It is, therefore, the object of the invention to provide a turbocharger system of the type incorporating two turbochargers which does provide a substantial boost pressure at substantially all engine speeds and enables the engine to produce a significantly enhanced power output at low engine speeds.